I. Field of the Invention
This invention relates to diamond tipped tungsten carbide inserts, the body of the insert being inserted into apertures formed in a body of a rock bit.
More specifically this invention relates to a means to improve the ability of the body of the insert to withstand shear forces encountered by the diamond cutter insert as it is utilized in a rock bit in an earth formation.
II. Description of the Prior Art
Diamond cutter discs as well as diamond insert stud blanks, for example, are fabricated from a tungsten carbide. A tungsten carbide substrate has a diamond layer sintered to a face of the substrate. The disc is then brazed to the stud body, the diamond layer being composed of a polycrystalline material. The synthetic polycrystalline diamond layer is manufactured by Megadiamond, a division of Smith International, Inc. located in Provo, Utah. The diamond insert produced by Megadiamond is known as the M-40 cutter.
The tungsten carbide disc with the synthetic polycrystalline diamond layer secured thereto is normally brazed to a tungsten carbide stud. The stud is designed to be pressed or bonded within the face of, for example, a drag type rock bit.
During the brazing certain elements such as boron silicon in the braze material leaches out the cobalt in the tungsten carbide stud body thereby forming a lower strength brittle alloy or a metallurgical "notch" or crack at the juncture of the braze and the tungsten carbide. This is especially detrimental where a braze fillet forms between the flat surface of the tungsten carbide insert body and the circumferential surface of the tungsten carbide substrate of the diamond disc. This rounded surface is perpendicular to the flat mounting surface over at least half of the diameter of the disc. This phenomenon creates a stress riser at the aforementioned filleted junction between the tungsten carbide substrate and the tungsten carbide body. The weakened juncture in the stud body limits the utility of the diamond cutter. The cutter insert will not withstand severe shear and tensile forces created when the bit is rotated in a borehole.
If the tungsten carbide stud body breaks or shears under shear or bending loads created when the drag bit cuts into a bottomhole formed in an earth formation, the broken cutting end of the stud then drops to the borehole bottom and destroys or damages the rest of the cutters on the bit thus shortening the life of the bit.
The prior art is therefore disadvantaged in that, as a result of the brazing process whereby the diamond cutter substrate is brazed to the end of a tungsten carbide body, leaching of the cobalt from the tungsten carbide body creates a multiplicity of metallurgical notches weakening the stud body at the aforementioned filleted juncture of the substrate and the body.
This invention directs itself to overcoming these weakened areas by redesigning the surfaces to be brazed thereby minimizing the affects of the leaching process thus strengthening the cutters tremendously.